With recent advancement of digital technologies, electronic hardware such as portable information devices and home information appliances have been developed to provide higher functionality. For this reason, demands for an increase in a capacity of a nonvolatile memory element, reduction in a write electric power in the memory element, reduction in write/read time in the memory element, and longer life of the memory element have been increasing.
Under the circumstances in which there are such demands, it is said that there is a limitation on miniaturization of an existing flash memory using a floating gate. Accordingly, in recent years, a novel nonvolatile memory element (resistance variable memory) using a resistance variable layer as a material of a memory section has attracted an attention.
The resistance variable memory includes a memory element having a very simple structure in which a resistance variable layer is sandwiched between electrodes. The resistance variable layer switches reversibly among plural resistance states having different resistance values in response to a predetermined electric pulse applied between electrodes. The plural resistance states are used to store numeric value data. Because of the simplicity in structure and operation, it is expected that the resistance variable memory can achieve further miniaturization, a higher speed and lower electric power consumption.
The materials used as the resistance variable layer are roughly classified into two kinds. One kind of materials are oxides of transition metals (Ni, Nb, Ti, Zr, Hf, Co, Fe, Cu, Cr, etc) disclosed in Patent document 1 and Non-patent documents 1 to 3. In particular, they are oxides (hereinafter referred to as oxygen-deficient oxides) which are deficient in oxygen content in terms of a stoichiometric composition. The other kind of materials are perovskite materials (Pr(1-x)CaXMnO3 (PCMO), LaSRMnO3 (LSMO), GdBaCoxOy (GBCO). Patent documents 2 and 3 and Non-patent document 4, and the like disclose techniques for using the latter materials as elements capable of storing binary data (two states of low-resistance state and high-resistance states) and as elements capable of storing multi-valued data of three or more values.
FIG. 35 is a view showing an example of resistance switching of an element comprising PCMO in response to electric pulses, which is disclosed in Patent document 2. As can be seen from FIG. 35, by applying an electric pulse with a predetermined polarity, voltage and pulse width, a predetermined number of times, to an element being in an initial state and having a resistance value of about 500Ω, the resistance value increases or decreases. The resistance value switches such that substantially continuous values are attainable. It is disclosed that by selectively using the three or more states having different resistance values and corresponding three or more different numeric values to the associated resistance values, it is possible to attain a multi-valued memory element.
FIG. 36 is a view showing the relationship between the resistance value of the nonvolatile memory element using the PCMO or the like, applied voltage and resistance value which is disclosed in Patent document 3. In the example shown in FIG. 36, each electric pulse is applied once. As can be seen from FIG. 36, the resistance value of the element switches substantially continuously according to the voltage value of the electric pulse applied. As in the configuration disclosed in Patent document 2, Patent document 3 discloses that it is possible to attain a multi-valued memory element.